1. Field of the Invention
This invention relates to a stabilized direct current (DC) power supply of the switching type for application to an electronic device. More particularly, it relates to a stabilized direct current (DC) power supply of the switching type for application to an electronic device which contains a circuit sensitive to noise and a power supply handling large power in a single cabinet.
2. Description of the Prior Art
The above-mentioned electronic device often adopts a stabilized direct current (DC) power supply of the switching type (hereinafter referred to as switching regulator) as a power supply for the internal circuitry. The switching regulator can provide a desired power supply efficiently because of its principles, and produces a great effect on reducing power consumption or heating of the device.
Referring now to FIGS. 1 to 4, the prior art is described by taking as an example an onboard tuner and compact disk (DC) player unit 1 contained in a single cabinet.
FIG. 1 is a block diagram centering around a power supply for an onboard tuner and CD player unit adopting a switching regulator, wherein numeral 100 is a power input terminal of a car battery power supply to the electronic device into which normally a voltage of about 13 V is fed. Numeral 101 is a tuner power supply for supplying stabilized DC power to a tuner circuit 102, numeral 103 is an external antenna of the device, and numeral 104 is an antenna input terminal which is connected to the tuner circuit 102.
Numeral 105 is a CD power supply for supplying the first stabilized power (herein, 7 V for description) to a CD player block and is a power supply circuit of the switching type. Output of this power supply is used as power of a spindle motor circuit which gives a rotation driving force to a disk (not shown).
Numeral 106 is a CD 5-V power supply for supplying the second stabilized power (herein, 5 V for description) to the CD player block. The CD 5-V power supply is a normal stabilized power supply (called a series regulator) using a linear amplifier.
Numeral 107 is a digital signal processing circuit, shown as a typical circuit block of a CD player, which demodulates signals read from a disk and corrects errors (detailed description of operation is omitted); the circuit receives power from the CD 5-V power supply 106 for operation.
Numeral 108 is a loading motor as a driving source of a disk loading and ejecting mechanism, and numeral 109 is a motor drive circuit which controls forward and reverse rotations of a loading motor 108 and uses output of the CD a power supply 105 as power supply for operation.
Numeral 110 is a centralized controller which consists of a microprocessor, etc., and controls tuner operation and CD player operation intensively. Numeral 111 is a tuner power supply control line for instructing output of the tuner power supply 101 to be turned ON or OFF and numeral 112 is a CD power supply control line for instructing output of the CD power supply 105 to be turned ON or OFF and is given the signal name V.sub.c for description. Numeral 113 represents loading control lines for instructing operation of the loading motor 108. All the control line signals are output from the centralized controller 110.
FIG. 2 is a schematic drawing of the disk loading mechanism of the tuner and CD player unit, wherein the same part is designated by the same reference numeral in FIG. 1. In FIG. 2, numeral 200 is a cabinet of the tuner and CD player unit, numeral 201 is a compact disk (hereinafter simply disk), and numeral 202 is an operation panel.
The disk 201 is mounted on a tray part 203 for conveying to the inside or outside of the cabinet 200.
Rotation of the loading motor 108 is propagated via a belt 204, a spur gear 205, and a rack part 206 fixed to the tray part 203 and the tray part can be moved to the inside or outside of the cabinet 200 in response to the rotation direction of the loading motor 108. Numeral 207 is a disk clamp mechanism provided to hold the disk 201 on a turn table (not shown) during reproduction operation (detailed description is omitted).
FIG. 3 is a drawing showing a specific configuration example of the CD power supply 105 shown in FIG. 1, wherein the same parts are designated by the same reference numerals in FIGS. 1 and 2. The switching regulator shown in FIG. 3 is a step-down regulator of the chopper type (output voltage is lower than input voltage). In FIG. 3, numeral 1 is a power input terminal into which voltage V.sub.i (which is 13 V in average and fluctuates from 10 V to 16 V or so) is fed, and numeral 2 is a power ON/OFF terminal to which the CD power supply control line 112 for controlling ON/OFF of regulator output is connected and the signal V.sub.c is applied. This example assumes that V.sub.c =5 V to perform normal operation and V.sub.c =0 V to instruct output stop. Numeral 3 is an output terminal of the power supply from which the voltage V.sub.o is output.
Numeral 4 is a reference voltage, numeral 5 is an error amplifier which detects and amplifies the difference between the reference voltage 4 and output voltage V.sub.o, numeral 6 is a saw tooth generator, and numeral 7 is a PWM converter which performs PWM conversion by comparing the saw tooth signal v3 from the saw tooth generator with the output vl of the error amplifier 5. Numeral 8 is a series transistor whose emitter is connected to the power input terminal 1 and whose collector to the output terminal 3 via a choke 9, numeral 10 is a diode, and numeral 11 is a smoothing capacitor connected between the connection point of the choke 9 and the output terminal 3 and ground; these are basic circuit elements of a switching regulator of the chopper type.
Numerals 12 and 13 are drive transistors and numerals 14 and 15 are current setting resistors; these make up a drive circuit which transmits output of the PWM converter 7 to the series transistor 8. The current setting resistor 14 is connected between the output terminal of the PWM converter 7 and the base of the drive transistor 12. Between the base of the series transistor 8 and ground, the drive transistor 13, the current setting resistor 15, and the drive transistor 12 are connected in series in order.
The power ON/OFF terminal 2 (voltage is V.sub.c) is V.sub.c is 0 V , the drive transistor 13 is completely cut off connected to the base of the drive transistor 13. When and at the same time, the series transistor 8 is also cut off, causing output of the power supply to become 0 V (OFF).
When V is 5 V , the drive transistor 13 operates as a common base amplifier. In this state, when output of the PWM converter 7 is low, the drive transistor 12 is turned OFF and at the same time, the drive transistor 13 and series transistor 8 are also turned OFF. When output of the PWM converter 7 is high, the drive transistor 12 is turned ON and a specific emitter current flows into the drive transistor 13; resultantly, the series transistor 8 is also turned ON.
FIGS. 4a-4b show the waveforms of the blocks of the stabilized DC power supply of the chopper type shown in FIG. 3. Using the drawing, operation of the description to follow assumes that the voltage V.sub.c of the circuitry shown in FIG. 3 will be described. The power ON/OFF terminal 2 is 5 V . FIG. 4a shows waveforms of output V3 of the saw tooth generator 6 (solid line) and of output V1 of the error amplifier 5 (broken line). It shows an example in which since load current lowers from the latter half of the time axis, the output voltage V.sub.o rises. The PWM converter 7 compares the two signals shown in FIG. 4a and outputs the signal V2 shown in FIG. 4b. FIGS. 4a and 4bshow that when the output voltage V.sub.o is high, the high duty of the output V2 of the PWM converter 7 lowers.
When the output V2 is high, the series transistor 8 is turned ON and during this period, the voltage V4 of the collector of the series transistor 8 becomes V.sub.I which is substantially equal to the input voltage v.sub.i, as shown in FIG. 4c. While the transistor is ON, the current I.sub.c of the choke 9 (broken line in FIG. 4d) and the collector current I.sub.tr of the serial transistor 8 (solid line in FIG. 4d) increase with the time.
On the other hand, when the series transistor 8 is OFF, the collector voltage V4 is dropped because of the nature of retaining the previous current of the chopper 9, and the collector voltage V4 is clamped at a place of slightly negative voltage because of function of the diode 10 (FIG. 4c). While the transistor is OFF, the choke current I.sub.c decreases with the time for a while as shown in FIG. 4d, but current supply is continued to the output side.
In the latter half of the time axis in FIG. 4, that is, when the output voltage V.sub.o rises the ON duty of the series transistor 8 lowers. Accompanying this, the average value of the choke current I.sub.c output through the choke 9 to the output terminal 3 also lowers as shown in FIG. 4d, and stabilization operation is performed so as to lower the output voltage V.sub.o. Generally, 20 kHz to 500 kHz are selected as switching frequencies at a stabilized power supply of the switching type. Recently, high-speed switching devices are provided and switching frequencies of 100 kHz or higher are often selected to obtain merits of miniaturization.
Operation of the former tuner and CD player unit which contains the switching regulator of the chopper type described above will be described below by using flowcharts shown in FIGS. 5(a) to 5(c).
To newly reproduce the disk 201, the disk 201 is mounted on the tray part 203. In FIGS. 5(a) , when reproduction start operation input by an operation key on the operation panel 202 (not shown) is received, in step S110 the centralized controller 110 sets the CD power control line 112 (signal name V.sub.c) high (that is, sets V.sub.c to 5 V) . This signal is fed into the CD power supply 105, switching regulator operation is performed, and the output ON state is entered. This energizes all circuit blocks related to the CD player. At the same time, the output OFF state is specified for the tuner power supply 101 through the tuner power supply control line 111.
Next, in step S120 the centralized controller 110 outputs a specific signal to the loading control line 113, drives the loading motor 108 through the motor drive circuit, brings the tray part 203 into the inside of the cabinet 200, and completes loading operation (loading operation processing).
After the loading operation terminates, signals are read from the disk 201 and reproduction operation is performed by other devices (not shown). Power required for the CD player block is almost supplied from the CD power supply 105. Since this power supply is of the switching type, extra heating from the power supply block can be minimized and a temperature rise in the inside of the cabinet 200 can be suppressed to be low.
Next, FIG. 5(b) is used to describe processing of the centralized controller 110 when tuner reception is instructed by operation (not shown) during disk reproduction. The centralized controller 110 instructs predetermined CD player block stop operation in step S210, and with the disk 201 mounted in the inside of the cabinet 200, finally placing the CD power supply 105 in the output OFF state in step S220. On the other hand, the tuner power supply 101 is changed to the output 0N state through the tuner power supply control line 111, enabling the tuner circuit to operate.
Next, FIG. 5(c) is used to describe processing of the centralized controller 110 when disk ejection operation (not shown) is performed during tuner reception. The centralized controller 110 places the CD power supply 105 in the output ON state in step S310 while continuing tuner reception operation. In step S320, the centralized controller drives the loading motor 108 in the reverse rotation to the loading time and ejects the tray part 203 or disk 201 (ejection processing) through the loading control line 113 and motor drive circuit 109. When the tray part 203 moves to a specific ejection end position, the centralized controller 110 instructs output of the CD power supply 105 to be turned OFF in step S330.
The following problem arises for a sequence of the operation steps of the tuner and CD player unit described above: If the CD power supply 105, that is, a power supply of the switching regulator type is operated for disk ejection operation, the power supply generates high-frequency noise at considerably large level. Specifically, the peripheral circuit is affected by electrostatic induction or electro-magnetic induction caused by pulse voltage and current of large amplitude or by direct invasion as a voltage drop at wiring impedance. Affection is liable to extend to the peripheral circuit because of recent high-frequency switching regulators.
The noise can be ignored unless the unit is in the tuner reception state; but in the reception state, noise is entered in the tuner circuit or antenna in the same cabinet and there is an extremely high risk the reception quality will lower. Depending on the situation, even in an acoustic system which contains a tuner in a separate cabinet, noise of switching power supply may cause the reception quality to be lowered.
To solve the problem, the following countermeasures should be adopted:
ol Seal the switching regulator up in a sealed case. PA1 o2 Provide a noise removing filter for the power input/output block, etc. PA1 o3 Seal the tuner block up in a sealed case. PA1 o4 Keep the tuner block apart from the switching regulator block. PA1 o5 Lower or change the switching frequency of the switching regulator. PA1 o6 Use a choke with less magnetic flux leakage.
If the noise problem is completely removed by combining these countermeasures easily and with a few number of parts and a slight cost increase, it is satisfactory. On the other hand, as an essential countermeasure, a series regulator with a normal line amplifier rather than a power supply of the switching type can be used to supply power to the motor drive circuit in order to solve the noise problem.
For this countermeasure, if a stabilized power supply of the switching type is adopted as the main power supply during reproduction operation of the CD player, a series regulator should be provided apart from it and at least when ejection operation is performed during tuner reception, only the series regulator should be operated for driving the loading motor.
Since the former stabilized DC power supply is configured as described above, if the switching regulator type is adopted, the countermeasures listed above in ol to o6 must be combined for noise countermeasures. In fact, a large number of circuits, mechanisms, and features must be installed.
In a narrow space such as an onboard device, there are many restrictions on layout design of circuit blocks and shield design, and a satisfactory result based upon the aforementioned measures often cannot be obtained.
In addition, if a series regulator is used to supply power to the motor drive circuit, and a power supply to drive a motor handles a large power and current, then a semiconductor device for allowing a large heat loss is required. Such a device, which takes a large form, requires extra circuit installation space and parts cost.
From the viewpoints of circuit installation space, parts cost, and heat loss, the stabilized DC power supply whose regulator circuit is switched between a switching regulator and series regulator in an alternative way, disclosed in Japanese Patent Laid-Open No. Sho 64-88612, etc., is preferable. However, since it is switched in response to the input voltage level without respect to a harmful influence of noise, the power supply requires noise countermeasures as described above.